In a rolling bearing, local permanent deformation may be generated in a contact part between each raceway surface of outer and inner rings and rolling elements in the case where an excessive static load is imparted on use or in the case where a large impact load is imparted during rotation at an extremely low number of rotation. The permanent deformation increases with an increase of the load and when the load exceeds a limit load (static rating load), smooth rotation is inhibited. For example, in the case of a ball bearing, maximum contact stress between the rolling elements and the raceway part of the outer and inner rings is regarded as about 4200 MPa. At the design of the ball bearing, the static rating load is determined in consideration of such a stress.
An improvement of the static rating load is one important problem for realizing miniaturization and weight saving of a rolling bearing.
On the other hand, when a rolling bearing rotates with receiving a load, a stress is repeatedly imparted to each raceway surface of outer and inner rings and rolling surfaces of the rolling elements. Therefore, when the rolling bearing is continuously used for a certain period of time, rolling fatigue is generated in the materials of the bearing constituent members constituting the rolling bearing and thus flaking is generated on the raceway surface and the rolling surfaces. The total number of rotation (referred to as a “rolling fatigue life”) of the rolling bearing until the flaking is generated on the raceway surface and the rolling surfaces is regarded as an index of changing the rolling bearing. The longer the rolling fatigue life is, the lesser the frequency of changing the rolling bearing can be.
As a rolling bearing having an improved rolling fatigue life, there has been known a rolling bearing provided with outer and inner rings where the total carbon amount in the surface layer ranging from the surface to a depth on which maximum shear stress acts is controlled to 1.0 to 1.6% by mass, the dissolved carbon amount in the matrix in the surface part ranging from the surface to a depth of 0.5 mm is controlled to 0.6 to 1.0% by mass, carbon compounds are precipitated in the surface part, the amount of the precipitated carbon compounds is controlled to 5 to 15% as a area ratio, and the particle diameter of the particles of the carbon compounds is controlled to 3 μm or less by subjecting a workpiece, which has been shaped by subjecting a high carbon chromium bearing steel such as JIS SUJ2 to machining or the like into a prescribed shape, to a carburizing treatment where the workpiece is heated at 840 to 870° C. for 3 hours or longer in a carburizing atmosphere of a carbon potential of 1.2 or more, is then rapidly cooled, and is further subjected to a tempering treatment (see Patent Document 1).
However, from the viewpoint of achieving miniaturization of components, use applications under severe use environments such as an increase of the load on the outer and inner rings and the rolling elements and high temperature on use have increased, so that a rolling bearing exhibiting higher performance has been required.
Moreover, in the case where a rolling bearing is used, for example, under foreign matter contamination conditions, the foreign matter is pressed to the inner and outer rings and the rolling elements that are bearing constituent members to form brinelling on the raceway surface of the inner and outer rings and the surfaces of the rolling elements in some cases. The surface damaged part such as the brinelling formed by the foreign matter becomes a starting point of fatigue flaking by stress concentration, which is one cause of lowering the life of the rolling bearing.
Therefore, it is desired to improve the life of the rolling bearing under the foreign matter contamination conditions and the like.
In order to improve the life of the rolling bearing, it has been proposed to obtain a bearing constituent member by subjecting a workpiece, which has been formed into a prescribed shape from a steel composed of SUJ2 that is a bearing steel, to a carbonitriding treatment.
Moreover, it has been proposed that an average particle diameter of particles composed of carbides in the surface layer part of a bearing constituent member of a rolling bearing is controlled to 0.5 μm or less, the area ratio of the particles composed of the carbides is controlled to 9 to 30%, Rockwell C hardness is controlled to 63 (Vickers hardness of 770) or more, the amount of residual austenite in the surface layer is controlled to 30 to 50% by volume by subjecting a workpiece, which has been formed into a prescribed shape from a steel containing 3.2% by mass or more to less than 5.0% by mass of chromium, less than 1.0% by mass of molybdenum, and less than 0.5% by mass of vanadium, to a carburizing treatment where the workpiece is heated at 850 to 930° C. in a carburizing atmosphere of a carbon potential of 1.2 to 1.5 and is hen rapidly cooled, and subsequently tempering the resultant workpiece (see Patent Document 2).